This invention relates to RF (radio frequency) directional couplers and more particularly to a device for detecting incident or reflected waves.
The detection of the power of incident or reflected propagating RF waves depends on sampling of RF line voltage and RF line current and comparing vectorially two current derivatives of the line voltage and line current. The prior art systems have very poor sensitivity in the nulling or substraction process of the line voltage and line current energy sample in the reverse or reflected power indicating mode. The result is spurious residual output when sampling a transmission line terminated in a matched load or loads departing slightly from the transmission line characteristic impedance, causing the line VSWR (Voltage Standing Wave Ratio) to depart from a value of one-to-one. Such prior art devices when used to turn back the drive of a transmitter as a function of deteriorating antenna load impedance provide signals that falsely turn back the transmitter drive, particularly on AM modulation. It is desirable not to reduce transmitter drive until antenna load mismatch approaches a higher value that would seriously damage or degrade the transmitter.